METHOD AND DEVICE FOR MEASURING THE CLEANING CAPACITY OF A CLEANING BATH

Abstract: Method and arrangement for measuring the cleaning capacity of a cleaning bath, the light absorbing ability of which is dependent on both the content of cleaning substances and the degree of impurity, as well as the temperature. The light absorbing ability of the solution is measured at at least two different selected temperatures and with varying quantity of added oil. The remaining cleaning capacity of the bath is determined as corresponding to the quantity of oil which is added to the bath when the difference in light absorption at the warm and the cold temperature has reduced to zero.

Description: Method and device for measuring the cleaning capacity of a cleaning bath TECHNICAL FIELD: The present invention relates to a method and an arrange ment for measuring the cleaning capacity of a cleaning bath according to the preamble of claim 1 and claim 5 respect ively. BACKGROUND OF THE INVENTION: Cleaning baths for various industrial processes attain progressively worsened properties after, for example, dipping of a large number of objects to be treated, where each object releases substances in the bath from an earlier process stage. One example of such an industrial process is surface treatment where metal surfaces are cleaned or pretreated before a surface treatment process. The quality of cleaning is critical for successful surface treatment. The cleaning may, for example, consist of removal of oils, cooling emulsions and particles from earlier machining steps. Within the car industry, degreasing of car bodies or body parts before phosphatising is a comprehensive cleaning process, where the quality of cleaning is of very great significance. A high capacity of each bath is desired, at the same time that a plurality of cleaning baths are included in the process which together contain a very large liquid volume. For economic and technical reasons, the cleaning solution is used for a relatively long time which varies depending on the quantity of goods to be cleaned and the degree of impurity. The cleaning process is operated until it is determined that the cleaning effect has sunk to a level at which it is no longer possible to guarantee good cleaning. The duration of use can be extended by adding cleaning chemicals during the process until it is determined that the cleaning solution has been entirely consumed and it is then disposed of at a purifying plant. The cleaning properties of the cleaning solution are gradually worsened for a number of reasons, including the fact that oil and dirt which are introduced will tie up the active cleaning substances, as well as due to the bath liquid from the previous stage being transferred together with the goods to be cleaned, in addition to the cleaning solution being removed on the goods after treatment. Since simple and reliable methods to control the quality of the cleaning bath are not available, the bath is normally discharged according to a previously determined schedule. In order to monitor the cleaning substance concentration in the bath, it has been proposed that the pH-value be measured and a titration with acid carried out to establish the buffer capacity of the bath. Cleaning baths normally contain tensides as the active cleaning substance, the concentration of which cannot, however, be measured in the stated manner to sufficient accuracy since alkalies included in the cleaning bath affect to too high a degree the total pH-value of the liquid. In order to reduce the quantity of discharged cleaning solution, in addition to chemical treatment of the bath some form of physical treatment such as, for example, filtration or centrifuging can take place, which results in the balance between the different components in the cleaning substance being changed, which implies that a need exists to measure the properties of the bath in order to be able to control the quality. SUMMARY OF THE INVENTION: The object of the present invention is to provide a method and an arrangement for measuring the cleaning capacity of a cleaning bath in such a manner that reliable information concerning changes in the properties of the bath is obtained. Said object is achieved by a method and an arrangement according to the present invention, the features of which are detailed in claim 1 and claim 5 respectively. BRIEF DESCRIPTION OF THE DRAWINGS: The invention will be described in greater detail in the following by way of example and with reference to the attached drawings, in which Fig. 1 shows schematically a detection arrangement accord ing to the invention, and Fig. 2 shows a diagram of the relation between light absorption and degree of impurity in a cleaning bath at different temperatures. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS: The present invention makes use of a special property of certain cleaning baths, namely that their ability to absorb light is, to a large extent, temperature dependent. One example of such a liquid bath is a cleaning bath which normally consists of substantially three sections, i.e. a tenside section which is the active substance for the cleaning process, a complex former the purpose of which is to bind ions (primarily responsible for the hardness of water) and which can support the function of the tenside, and finally alkali the purpose of which is to create a favourable pH-value for the process. In the tenside section a non-ion tenside can, for example, be included which when mixed in water gives a clear solution at room temperature, but at a certain temperature change turbidity of the solution arises. About this temperature, which is called the tenside's turbidity point, the tenside exhibits its best cleaning effect. The degree of turbidity also depends on the quantity of tenside in the bath and the relationship is, in principle, linear over a certain concentration, the so-called critical micelle formation concentration CMC. The turbidity in a process bath also increases, however, with time due to admixture of foreign substances such as oil and dirt from dipped articles to be cleaned, for which reason a simple direct measurement of turbidity provides insufficient information. According to the invention, it has been noted that the difference in turbidity when measuring at two temperature levels when using certain surface-active substances is dependent on the concentration of these substances. When using tensides, the difference in turbidity above and below the tenside's turbidity point is thus primarily dependent on the tenside concentration and thus the cleaning properties of the cleaning solution. At a certain level of impurity, this difference in turbidity reduces to zero which has been shown to correspond to the condition at which it is accepted that the cleaning solution is consumed, i.e. all active substance is tied up by impurities and thus inactive. The invention is based on a detection arrangement which measures the turbidity in an optical manner by measuring the light absorption or transmission of the cleaning solution. An example of such a detection arrangement is shown schematically in Fig. 1. The arrangement consists primarily of a light source 1 which in the shown example is a laser which emits a very focused beam 2. Further included in the arrangement is a measuring cell 3 in the form of, for example, a cuvette, i.e. a transparent container with, for example, flat glass or plastic walls arranged to contain a certain volume of a sample of cleaning solution. The measuring cell provides a set transmission path for the beam. A heating source 4 is also included in the arrangement for heating the solution in the measuring cell to a selected temperature. The heat source is adjustable between a switched-off position and a chosen power level and is thus suitably provided with a thermostat which may be connected to a temperature sensor in the measuring cell. In a simpler embodiment, a thermometer can be used to indicate the temperature in the measuring cell. A light detector 5 is also included in the detection arrangement. The detector 5 may possibly be adjusted to different angles and is arranged to emit a detection signal, suitably an electrical signal, to a calculating and display unit 6 for optical or graphic presentation of the received light and thus the transmission/absorption ability of the cleaning solution. This can thus be given as a function of the quantity of added impurity, in this case the concent: tion of oil, at two temperature levels. The cleaning soS ion in the present example contains a tenside as active cleaning substance, whereby a first temperature is selected under the turbidity point of the tenside, for example 30"C, and a second temperature level above the turbidity point, such as 50"C. A practical execution of the detection can for example include that a first measurement be carried out on a sample of the cleaning solution in the measuring cell at the first temperature, 30"C, starting with a degree of impurity, i.e. oil content in the shown example, which occurs in the cleaning solution when the sample is taken. Thereafter oil is gradually added via, for example, titration during continued measuring, for example continuous measuring or at fixed intervals, up to an estimated concentration of oil. Thereafter, a second sample is taken which is heated to the higher temperature of 50"C, whereat the measurement cycle is repeated during gradual addition of oil in a predetermined quantity until the same value of the light absorption/transmission is detected in the display unit 6. With more sophisticated equipment, two measuring cells can be detected in parallel with each having a light beam of the same light intensity. The sample in one of the measuring cells is heated to the lower temperature and the other measuring cell is heated to the higher temperature and two detectors each detect the transmitted light beam which is received in either a display unit for each beam or a common display unit which displays the difference in the light absorption/transmission. The quantity of oil which is added so that the two curves according to Fig. 2 meet, i.e. when the difference in light absorption is equal to zero, is a measurement of the remaining cleaning capacity of the cleaning solution or, conversely, the proportion of maximum cleaning capacity. Example 1 In a bath of a dip degreasing process, tenside is primarily consumed due to the fact that it forms molecule aggregates in the water solution, so-called micelles. In order to determine for how long such a bath can be used, as well as its condition, the bath can be controlled in the following manner: A sample is taken and the light absorption is measured above and below the turbidity point. Thereafter, a quantity of oil is added which corresponds to a certain operating time, whereafter the experiment is repeated. If the difference in the light absorption at the two temperatures exceeds 10%, the bath is approved for continued operation. The tests can thereafter be repeated after addition of more oil. If the difference does not arise, the bath can be additionally charged or replaced. In order to determine a suitable quantity of additional charge, the additional charge chemicals, for example a tenside mixture, is added to the warm sample until the turbidity at the higher temperature is observed. Example 2 In a spray degreasing process, a cleaning chemical is used which forms an unstable emulsion which will separate when the solution is not stirred. This is then used to remove the washed away oil in gravimetric oil separating equipment. However a certain amount of tenside which has dissolved itself in the oil phase will also be separated. A sample is taken and the light absorption measured above and below the turbidity point respectively. Thereafter, a quantity of oil corresponding to a certain operating time is added during stirring. The solution is allowed to separate and the water phase is analyzed as above. If the difference in light absorption at the two temperatures exceeds 10%, the bath is approved for continued operation. The test can then be repeated after new oil is added. If the difference does not arise, the bath can be additionally charged or replaced. In order to determine a suitable quantity of additional charge, the additional charge chemical, for example a tenside mixture, is added to the warm sample until the turbidity at the higher temperature is observed. This information can also be used to adjust the additional charge equipment which is often present in these cleaning processes. Example 3 With a view to increasing the useful life of degreasing baths and to reuse chemicals in spent baths, different separation techniques are used in which a bath is cleaned from impurities such as oil and particles, whilst the water which still contains a lot of the cleaning chemicals can be used for further operation. However, a large portion of the tensides is often separated. In such a process, the additional charge method is critical and the additional charge should be made with separate tenside mixtures. In order to determine a suitable quantity of additional charge, a bath sample is taken of the recycled solution which can be analyzed with respect to the turbidity above and below the turbidity point, after which a quantity of oil corresponding to a certain operating time is added. A tenside mixture is then added to the sample until the difference in turbidity exceeds 10%. The bath can then be additionally charged with a corresponding quantity of tenside mixture. The invention is not restricted to the embodiments described above and shown in the drawings, but can be varied within the scope of the appended claims. For example, cleaning bath other than baths for cleaning can be envisaged. In addition, the active substances other than tensides having the same properties, i.e. a temperature dependent light absorption ability, can be employed to measure the active properties of a liquid. The detection equipment can make use of light other than laser light. It is obvious that the absorption property cannot be determined directly by the quantity of light received by the detector, but instead information concerning the quantity of light received by the measuring from the light source must be taken into consideration in order to provide an absolute value. According to the invention, however, in order to detect the difference it is sufficient for remaining similar conditions to measure the quantity of light received by the detector. The expression impurities and degree of impurity is intended to cover all types of substances which do not form a part of the original solution.

Claims: Claims: 1. Method for measuring the cleaning capacity of a cleaning bath, the light absorbing ability of which is dependent on the content of cleaning substances, as well as the degree of impurity and the temperature, c h a r a c t e r i z e d i n that the light absorbing ability of the solution is measured at at least two different selected temperatures, and in that the difference in the light absorption ability at said temperatures is detected as a value of the remaining cleaning capacity or, alternatively, a proportion of the maximum cleaning capacity. 2. Method according to claim 1, c h a r a c t e r i z e d i n that impurities are gradually added to a sample for each selected temperature during continuous measurement of the light absorbing ability of the respective samples until the difference in the light absorbing ability is reduced to a chosen value, for example zero, during measuring of the quantity of added impurities, which is thereby directly related to the cleaning capacity of the bath. 3. Method according to claim 1 and 2 for measuring the cleaning capacity of a cleaning bath intended for cleaning of material, the active cleaning substance of which contains at least a tenside with known turbidity point, c h a r a c t e r i z e d i n that said measuring occurs both at a first temperature which is beneath the turbidity point of the tenside, as well as at a second temperature which lies above the turbidity point of the tenside. 4. Method according to claim 3, c h a r a c t e r i z e d i n that a first sample of said cleaning bath is illuminated at said first temperature, in that a second or the same sample of said cleaning bath is illuminated at said second temperature and that the difference between the absorption ability of two samples is detected during the addition of a controlled quantity of impurity of predetermined type, for example oil, until the difference has reduced to a predetermined value, for example zero, whereby the quantity of added impurity is measured as a measure of the remaining cleaning capacity of the cleaning solution. 5. Device for measuring the cleaning capacity of a cleaning bath, the light absorbing ability of which is dependent on the quantity of cleaning substances, the degree of impurity and the temperature, c h a r a c t e r i z e d b y at least one light source (1) for illuminating the cleaning bath or a sample thereof, a heat source (4) to bring the solution to at least two different temperatures, and a light detector (5) for measuring the light absorption in the solution at the different temperatures. 6. Device according to claim 5, c h a r a c t e r i z e d i n that a calculating and display unit (6) is included in the arrangement and is adapted to receive a signal indicating the measured light absorption and to calculate and display the difference in light absorption as a measure of the remaining cleaning capacity.

Inventors: KLOO HENRIK, WAAAAG AAKE, KLOO, HENRIK, WAAAAG, AAKE
Applicants: KLOO HENRIK, VOLVO AB, WAAAAG AAKE, AB VOLVO, KLOO, HENRIK, WAAAAG, AAKE
Application year: 1994
Main classification: G01N21
IPC codes: G01N33